The present invention relates to a directional coupler, and more particularly, to arrangements and structures of conductor patterns for accomplishing both of reduction in size and height and good electrical characteristics of the directional coupler.
A directional coupler (hereinafter simply referred to as the “coupler”) which has a function of branching or combining high frequency power propagated on a transmission line has become an indispensable component for designing a transmitter circuit for a variety of wireless communication devices such as portable telephones, wireless LAN communication device, communication devices based on Bluetooth (registered tradename) standard, and the like.
The coupler comprises a first line which has a first port at one end and a second port at the other end, respectively, and a second line which has a third port on one end and a fourth port on the other end, respectively. The first line and second line are disposed in close proximity to each other such that they are electromagnetically coupled to each other.
Such a coupler comprising coupling lines can be used as detector means for monitoring a transmitter circuit of a communication device for the level of a transmitted signal. Specifically, the coupler is inserted between a power amplifier (PA) for amplifying the transmitted signal and an antenna. The transmitted signal from PA is inputted to a first line (primary line) through a first port (input port) of the coupler, and is then outputted from a second port (output port) toward the antenna. In this event, part of the transmitted signal propagating through the first line is extracted through a second line (secondary line) which electromagnetically couples to the first line, and outputted from a third port (coupling port) to an automatic output control circuit (APC circuit) as a monitor signal. The APC circuit controls the gain of PA such that PA provides a constant output in accordance with the level of the monitor signal (i.e., the level of the transmitted signal). Such a PA feedback control enables the transmission output to be stabilized.
The coupler can also divide high-frequency power to two lines with a phase difference of 90°, or can combine high-frequency power from two lines with a phase difference of 90°. As such, the coupler can also be used, for example, for a differential power amplifier as an input divider or as an output combiner. Specifically, the coupler can be applied with a transmitted signal from the first port, divide the transmitted signal into two halves, and output the halves from the second port and third port, respectively, with a phase difference of 90°. In this way, the coupler can be used for a differential power amplifier as an input divider. Alternatively, the coupler can be applied with high-frequency signals with a phase difference of 90° from the second port and third port, respectively, combine these signals, and deliver the resulting single signal from the first port. In this way, the coupler can also be used for a differential power amplifier as an output combiner.
Further, the following patent documents disclose such couplers:    Patent Document 1: JP-A-2002-280810; and    Patent Document 2: JP-A-8-191206.